This invention relates to electro-optic displays and to materials, especially adhesive compositions and binders, for use therein. This invention relates in part to adhesive compositions and binders with electrical and other properties which render them especially suitable for use in electro-optic displays. Certain adhesive compositions of the present invention may also be useful in applications other than electro-optic displays. This invention also relates to materials and processes for lamination of electro-optic displays.
Electro-optic displays comprise a layer of electro-optic material, a term which is used herein in its conventional meaning in the art to refer to a material having first and second display states differing in at least one optical property, the material being changed from its first to its second display state by application of an electric field to the material. The optical property is typically color perceptible to the human eye, but may be another optical property, such as optical transmission, reflectance, luminescence or, in the case of displays intended for machine reading, pseudo-color in the sense of a change in reflectance of electromagnetic wavelengths outside the visible range.
The electro-optic displays of the present invention typically contain an electro-optic material which is a solid in the sense that the electro-optic material has solid external surfaces, although the material may, and often does, have internal liquid- or gas-filled spaces, and to methods for assembling displays using such an electro-optic material. Such displays using solid electro-optic materials may hereinafter for convenience be referred to as “solid electro-optic displays”. Thus, the term “solid electro-optic displays” includes rotating bichromal member displays (see below), encapsulated electrophoretic displays, microcell electrophoretic displays and encapsulated liquid crystal displays.
The terms “bistable” and “bistability” are used herein in their conventional meaning in the art to refer to displays comprising display elements having first and second display states differing in at least one optical property, and such that after any given element has been driven, by means of an addressing pulse of finite duration, to assume either its first or second display state, after the addressing pulse has terminated, that state will persist for at least several times, for example at least four times, the minimum duration of the addressing pulse required to change the state of the display element. It is shown in published U.S. patent application Ser. No. 2002/0180687 that some particle-based electrophoretic displays capable of gray scale are stable not only in their extreme black and white states but also in their intermediate gray states, and the same is true of some other types of electro-optic displays. This type of display is properly called “multi-stable” rather than bistable, although for convenience the term “bistable” may be used herein to cover both bistable and multi-stable displays.
Several types of electro-optic displays are known. One type of electro-optic display is a rotating bichromal member type as described, for example, in U.S. Pat. Nos. 5,808,783; 5,777,782; 5,760,761; 6,054,071 6,055,091; 6,097,531; 6,128,124; 6,137,467; and 6,147,791 (although this type of display is often referred to as a “rotating bichromal ball” display, the term “rotating bichromal member” is preferred as more accurate since in some of the patents mentioned above the rotating members are not spherical). Such a display uses a large number of small bodies (typically spherical or cylindrical) which have two or more sections with differing optical characteristics, and an internal dipole. These bodies are suspended within liquid-filled vacuoles within a matrix, the vacuoles being filled with liquid so that the bodies are free to rotate. The appearance of the display is changed to applying an electric field thereto, thus rotating the bodies to various positions and varying which of the sections of the bodies is seen through a viewing surface. This type of electro-optic medium is typically bistable.
Another type of electro-optic display uses an electrochromic medium, for example an electrochromic medium in the form of a nanochromic film comprising an electrode formed at least in part from a semi-conducting metal oxide and a plurality of dye molecules capable of reversible color change attached to the electrode; see, for example O'Regan, B., et al., Nature 1991, 353, 737; and Wood, D., Information Display, 18(3), 24 (Mar. 2002). See also Bach, U., et al., Adv. Mater., 2002, 14(11), 845. Nanochromic films of this type are also described, for example, in U.S. Pat. Nos. 6,301,038; 6,870.657; and 6,950,220. This type of medium is also typically bistable.
Another type of electro-optic display is an electro-wetting display developed by Philips and described in Hayes, R. A., et al., “Video-Speed Electronic Paper Based on Electrowetting”, Nature, Vol. 425, pages 383-385 (25 Sep. 2003). It is shown in copending Application Ser. No. 10/711,802, filed Oct. 6, 2004 (Publication No. 2005/0151709), that such electro-wetting displays can be made bistable.
Another type of electro-optic display, which has been the subject of intense research and development for a number of years, is the particle-based electrophoretic display, in which a plurality of charged particles move through a fluid under the influence of an electric field. Electrophoretic displays can have attributes of good brightness and contrast, wide viewing angles, state bistability, and low power consumption when compared with liquid crystal displays. Nevertheless, problems with the long-term image quality of these displays have prevented their widespread usage. For example, particles that make up electrophoretic displays tend to settle, resulting in inadequate service-life for these displays.
As noted above, electrophoretic media require the presence of a fluid. In most prior art electrophoretic media, this fluid is a liquid, but electrophoretic media can be produced using gaseous fluids; see, for example, Kitamura, T., et al., “Electrical toner movement for electronic paper-like display”, IDW Japan, 2001, Paper HCS1-1, and Yamaguchi, Y., et al., “Toner display using insulative particles charged triboelectrically”, IDW Japan, 2001, Paper AMD4-4). See also U.S. Patent Publication No. 2005/0001810; European Patent Applications 1,462,847; 1,482,354; 1,484,635; 1,500,971; 1,501,194; 1,536,271; 1,542,067; 1,577,702; 1,577,703; and 1,598,694; and International Applications WO 2004/090626; WO 2004/079442; and WO 2004/001498. Such gas-based electrophoretic media appear to be susceptible to the same types of problems due to particle settling as liquid-based electrophoretic media, when the media are used in an orientation which permits such settling, for example in a sign where the medium is disposed in a vertical plane. Indeed, particle settling appears to be a more serious problem in gas-based electrophoretic media than in liquid-based ones, since the lower viscosity of gaseous suspending fluids as compared with liquid ones allows more rapid settling of the electrophoretic particles.
Numerous patents and applications assigned to or in the names of the Massachusetts Institute of Technology (MIT) and E Ink Corporation have recently been published describing encapsulated electrophoretic media. Such encapsulated media comprise numerous small capsules, each of which itself comprises an internal phase containing electrophoretically-mobile particles suspended in a liquid suspending medium, and a capsule wall surrounding the internal phase. Typically, the capsules are themselves held within a polymeric binder to form a coherent layer positioned between two electrodes. Encapsulated media of this type are described, for example, in U.S. Pat. Nos. 5,930,026; 5,961,804; 6,017,584; 6,067,185; 6,118,426; 6,120,588; 6,120,839; 6,124,851; 6,130,773; 6,130,774; 6,172,798; 6,177,921; 6,232,950; 6,249,271; 6,252,564; 6,262,706; 6,262,833; 6,300,932; 6,312,304; 6,312,971; 6,323,989; 6,327,072; 6,376,828; 6,377,387; 6,392,785; 6,392,786; 6,413,790; 6,422,687; 6,445,374; 6,445,489; 6,459,418; 6,473,072; 6,480,182; 6,498,114; 6,504,524; 6,506,438; 6,512,354; 6,515,649; 6,518,949; 6,521,489; 6,531,997; 6,535,197; 6,538,801; 6,545,291; 6,580,545; 6,639,578; 6,652,075; 6,657,772; 6,664,944; 6,680,725; 6,683,333; 6,704,133; 6,710,540; 6,721,083; 6,724,519; 6,727,881; 6,738,050; 6,750,473; 6,753,999; 6,816,147; 6,819,471; 6,822,782; 6,825,068; 6,825,829; 6,825,970; 6,831,769; 6,839,158; 6,842,167; 6,842,279; 6,842,657; 6,864,875; 6,865,010; 6,866,760; 6,870,661; 6,900,851; 6,922,276; 6,950,200; 6,958,848; 6,967,640; 6,982,178; 6,987,603; 6,995,550; 7,002,728; 7,012,600; 7,012,735; 7,023,430; 7,030,412; 7,030,854; 7,034,783; 7,038,655; 7,061,663; 7,071,913; 7,075,502; and 7,075,703; and U.S. Patent Applications Publication Nos. 2002/0060321; 2002/0090980; 2002/0113770; 2002/0180687; 2003/0011560; 2003/0102858; 2003/0151702; 2003/0222315; 2004/0014265; 2004/0075634; 2004/0094422; 2004/0105036; 2004/0112750; 2004/0119681; 2004/0136048; 2004/0155857; 2004/0180476; 2004/0190114; 2004/0196215; 2004/0226820; 2004/0239614; 2004/0252360; 2004/0257635; 2004/0263947; 2005/0000813; 2005/0001812; 2005/0007336; 2005/0012980; 2005/0017944; 2005/0018273; 2005/0024353; 2005/0062714; 2005/0067656; 2005/0078099; 2005/0099672; 2005/0105159; 2005/0105162; 2005/0122284; 2005/0122306; 2005/0122563; 2005/0122564; 2005/0122565; 2005/0134554; 2005/0146774; 2005/0151709; 2005/0152018; 2005/0152022; 2005/0156340; 2005/0168799; 2005/0179642; 2005/0190137; 2005/0212747; 2005/0213191; 2005/0219184; 2005/0253777; 2005/0270261; 2005/0280626; 2006/0007527; 2006/0023296; 2006/0024437; and 2006/0038772; and International Applications Publication Nos. WO 00/38000; WO 00/36560; WO 00/67110; and WO 01/07961; and European Patents Nos. 1,099,207 B1; and 1,145,072 B1.
Many of the aforementioned patents and applications recognize that the walls surrounding the discrete microcapsules in an encapsulated electrophoretic medium could be replaced by a continuous phase, thus producing a so-called “polymer-dispersed electrophoretic display” in which the electrophoretic medium comprises a plurality of discrete droplets of an electrophoretic fluid and a continuous phase of a polymeric material, and that the discrete droplets of electrophoretic fluid within such a polymer-dispersed electrophoretic display may be regarded as capsules or microcapsules even though no discrete capsule membrane is associated with each individual droplet; see for example, the aforementioned U.S. Pat. No. 6,866,760. Accordingly, for purposes of the present application, such polymer-dispersed electrophoretic media are regarded as sub-species of encapsulated electrophoretic media.
A related type of electrophoretic display is a so-called “microcell electrophoretic display”. In a microcell electrophoretic display, the charged particles and the fluid are not encapsulated within microcapsules but instead are retained within a plurality of cavities formed within a carrier medium, typically a polymeric film. See, for example, International Application Publication No. WO 02/01281, and published US Application No. 2002/0075556, both assigned to Sipix Imaging, Inc. Hereinafter, the term “microcavity electrophoretic display” may be used to cover both encapsulated and microcell electrophoretic displays.
An encapsulated electrophoretic display typically does not suffer from the clustering and settling failure mode of traditional electrophoretic devices and provides further advantages, such as the ability to print or coat the display on a wide variety of flexible and rigid substrates. (Use of the word “printing” is intended to include all forms of printing and coating, including, but without limitation: premetered coatings such as patch die coating, slot or extrusion coating, slide or cascade coating, curtain coating; roll coating such as knife over roll coating, forward and reverse roll coating; gravure coating; dip coating; spray coating; meniscus coating; spin coating; brush coating; air knife coating; silk screen printing processes; electrostatic printing processes; thermal printing processes; inkjet printing processes; and other similar techniques.) Thus, the resulting display can be flexible. Further, because the display medium can be printed (using a variety of methods), the display itself can be made inexpensively.
Although electrophoretic media are often opaque (since, for example, in many electrophoretic media, the particles substantially block transmission of visible light through the display) and operate in a reflective mode, many electrophoretic displays can be made to operate in a so-called “shutter mode” in which one display state is substantially opaque and one is light-transmissive. See, for example, the aforementioned U.S. Pat. Nos. 6,130,774 and 6,172,798, and 5,872,552; 6,144,361; 6,271,823; 6,225,971; and 6,184,856. Dielectrophoretic displays, which are similar to electrophoretic displays but rely upon variations in electric field strength, can operate in a similar mode; see U.S. Pat. No. 4,418,346.
Other types of electro-optic materials, for example, polymer-dispersed liquid crystal, may also be used in some of the displays of the present invention.
In addition to the layer of electro-optic material, an electro-optic display normally comprises at least two other layers disposed on opposed sides of the electro-optic material, one of these two layers being an electrode layer. In most such displays both the layers are electrode layers, and one or both of the electrode layers are patterned to define the pixels of the display. For example, one electrode layer may be patterned into elongate row electrodes and the other into elongate column electrodes running at right angles to the row electrodes, the pixels being defined by the intersections of the row and column electrodes. Alternatively, and more commonly, one electrode layer has the form of a single continuous electrode and the other electrode layer is patterned into a matrix of pixel electrodes, each of which defines one pixel of the display. In another type of electro-optic display, which is intended for use with a stylus, print head or similar movable electrode separate from the display, only one of the layers adjacent the electro-optic layer comprises an electrode, the layer on the opposed side of the electro-optic layer typically being a protective layer intended to prevent the movable electrode damaging the electro-optic layer.
The manufacture of a three-layer electro-optic display normally involves at least one lamination operation. For example, in several of the aforementioned MIT and E Ink patents and applications, there is described a process for manufacturing an encapsulated electrophoretic display in which an encapsulated electrophoretic medium comprising capsules in a binder is coated on to a flexible substrate comprising indium tin oxide or a similar conductive coating (which acts as an one electrode of the final display) on a plastic film, the capsules/binder coating being dried to form a coherent layer of the electrophoretic medium firmly adhered to the substrate. Separately, a backplane, containing an array of pixel electrodes and an appropriate arrangement of conductors to connect the pixel electrodes to drive circuitry, is prepared. To form the final display, the substrate having the capsule/binder layer thereon is laminated to the backplane using a lamination adhesive. (A very similar process can be used to prepare an electrophoretic display useable with a stylus or similar movable electrode by replacing the backplane with a simple protective layer, such as a plastic film, over which the stylus or other movable electrode can slide.) In one preferred form of such a process, the backplane is itself flexible and is prepared by printing the pixel electrodes and conductors on a plastic film or other flexible substrate. The obvious lamination technique for mass production of displays by this process is roll lamination using a lamination adhesive. Similar manufacturing techniques can be used with other types of electro-optic displays. For example, a microcell electrophoretic medium or a rotating bichromal member medium may be laminated to a backplane in substantially the same manner as an encapsulated electrophoretic medium.
In the processes described above, the lamination of the substrate carrying the electro-optic layer to the backplane may advantageously be carried out by vacuum lamination. Vacuum lamination is effective in expelling air from between the two materials being laminated, thus avoiding unwanted air bubbles in the final display; such air bubbles may introduce undesirable artifacts in the images produced on the display. However, vacuum lamination of the two parts of an electro-optic display in this manner imposes stringent requirements upon the lamination adhesive used, especially in the case of a display using an encapsulated electrophoretic medium. The lamination adhesive must have sufficient adhesive strength to bind the electro-optic layer to the layer (typically an electrode layer) to which it is to be laminated, and in the case of an encapsulated electrophoretic medium, the adhesive should also have sufficient adhesive strength to assist in mechanically holding the capsules together. If the electro-optic display is to be of a flexible type (and one of the important advantages of rotating bichromal member and encapsulated electrophoretic displays is that they can be made flexible), the adhesive must have sufficient flexibility not to introduce defects into the display when the display is flexed. The lamination adhesive must have adequate flow properties at the lamination temperature to ensure high quality lamination, and in this regard, the demands of laminating encapsulated electrophoretic and some other types of electro-optic media are unusually difficult; the lamination has to be conducted at a temperature of not more than about 110° C. since the medium cannot be exposed to substantially higher temperatures without damage, but the flow of the adhesive must cope with the relatively uneven surface of the capsule-containing layer, the surface of which is rendered irregular by the underlying capsules. The lamination temperature should indeed be kept as low as possible, and room temperature lamination would be ideal, but no commercial adhesive has been found which permits such room temperature lamination. The lamination adhesive must be chemically compatible with all the other materials in the display. Solvent-based lamination adhesives should be avoided; it has been found (although this does not appear to have been described in the literature), that any solvent left behind in the adhesive after lamination has a strong tendency to introduce undesirable contaminants into the electro-optic medium.
As discussed in detail in the aforementioned U.S. Pat. No. 6,831,769, a lamination adhesive used in an electro-optic display must meet certain electrical criteria, and this introduces considerable problems in the selection of the lamination adhesive. Commercial manufacturers of lamination adhesives naturally devote considerable effort to ensuring that properties, such as strength of adhesion and lamination temperatures, of such adhesives are adjusted so that the adhesives perform well in their major applications, which typically involve laminating polymeric and similar films. However, in such applications, the electrical properties of the lamination adhesive are not relevant, and consequently the commercial manufacturers pay no heed to such electrical properties. Indeed, E Ink researchers have observed substantial variations (of up to several fold) in certain electrical properties between different batches of the same commercial lamination adhesive, presumably because the manufacturer was attempting to optimize non-electrical properties of the lamination adhesive (for example, resistance to bacterial growth) and was not at all concerned about resulting changes in electrical properties.
However, in electro-optic displays, in which the lamination adhesive is normally located between the electrodes which apply the electric field needed to change the electrical state of the electro-optic medium, the electrical properties of the adhesive become crucial. As will be apparent to electrical engineers, the volume resistivity of the lamination adhesive becomes important, since the voltage drop across the electro-optic medium is essentially equal to the voltage drop across the electrodes, minus the voltage drop across the lamination adhesive. If the resistivity of the adhesive layer is too high, a substantial voltage drop will occur within the adhesive layer, requiring an increase in voltage across the electrodes. Increasing the voltage across the electrodes in this manner is undesirable, since it increases the power consumption of the display, and may require the use of more complex and expensive control circuitry to handle the increased voltage involved. On the other hand, if the adhesive layer, which extends continuously across the display, is in contact with a matrix of electrodes, as in an active matrix display, the volume resistivity of the adhesive layer should not be too low, or lateral conduction of electric current through the continuous adhesive layer may cause undesirable cross-talk between adjacent electrodes. Also, since the volume resistivity of most materials decreases rapidly with increasing temperature, if the volume resistivity of the adhesive layer is too low, the performance of the display at temperatures substantially above room temperature is adversely affected. For these reasons, there is an optimum range of lamination adhesive resistivity values for use with any given electro-optic medium, this range varying with the resistivity of the electro-optic medium. The volume resistivities of encapsulated electrophoretic media are typically around 1010 ohm cm, and the resistivities of other electro-optic medium are usually of the same order of magnitude. Accordingly, the volume resistivity of the lamination adhesive should normally be around 108 to 1012 ohm cm, and preferably about 109 to 1011 ohm cm, at the operating temperature of the display, typically around 20° C.
While it may be apparent that there should be a relationship between the volume resistivities of the electro-optic medium and the lamination adhesive used in an electro-optic display, it has been discovered, as noted in the aforementioned U.S. Pat. No. 6,831,769, that other problems which have been observed in the operation of electro-optic displays are attributable to the electrical and related properties of the lamination adhesive. For example, although the number of commercial materials which can meet most of the previously discussed, rather disparate requirements for a lamination adhesive for use in an electro-optic display is small, in practice it has been found that a small number of water-dispersed urethane emulsions, primarily polyester-based urethane emulsions, do appear to have most of the requisite properties. However, although these materials perform well when the displays are first produced, after the resultant displays have been operated for substantial periods of time (of the order of hundreds of hours) at room temperature, or stored for a similar period, the performance of the display suffers substantial degradation. This degradation first manifests itself as reduced white state reflectivity and slower or incomplete switching of the electro-optic medium, especially in areas where the lamination adhesive is thickest; the thickness of the lamination adhesive may vary across the display both because of a non-planar electro-optic layer, as for example in an encapsulated electrophoretic medium where the spherical or ellipsoidal capsules introduce deviations from planarity, and/or because the manufacturing process normally used to produce the electrode matrix in such displays produces a non-planar surface on the electrode matrix. This degradation increases at lower temperatures (10° C. or below) and with time, so that after long periods the switching of the whole display is affected at room temperature. This degradation in optical performance with time is an important factor in limiting the service life of the displays.
The aforementioned U.S. Pat. No. 6,831,769 teaches that the afore-mentioned degradation in performance is caused, at least in part, by changes in the volume resistivity of the lamination adhesive, and that this performance degradation of electrophoretic displays can be reduced or eliminated, and the service life of such displays increased, by using an adhesive the resistivity of which does not vary greatly with time; it appears that similar effects are produced in other types of electro-optic displays. The use of such an adhesive has also been found to improve the performance of the displays at low temperature, as manifested by improved reflectance in the light optical state of the display.
Accordingly, the aforementioned U.S. Pat. No. 6,831,769 describes electro-optic displays in which the composition of the lamination adhesive is controlled so that the optical characteristics of the display do not change rapidly with time, so that the displays have an improved operating lifetime. More specifically, this patent teaches that the lamination adhesive should possess one or more of the following characteristics:                (a) having a volume resistivity, measured at 10° C., which does not change by a factor of more than about 3 after being held at 25° C. and 45 per cent relative humidity for 1000 hours;        (b) having a peel strength from an electrode material in contact with the lamination adhesive of at least about 2 lb/inch;        (c) the volume resistivity of the lamination adhesive changes by a factor of less than about 10 within a range of 10 to 90 per cent relative humidity and over a temperature range of 10 to 50° C.;        (d) the lamination adhesive has a thickness in the range of about 10 to about 20 μm;        (e) the lamination adhesive has a shear modulus at 120° C. of not more than about 1 megaPascal;        (f) the product of the dielectric constant and the volume resistivity of the lamination adhesive is not greater than the product of the dielectric constant and the volume resistivity of the electro-optic medium within a range of 10 to 90 per cent relative humidity and over a temperature range of 10 to 50° C.;        (g) comprising an ultra-violet stabilizer;        (h) comprising a light absorbing material;        (i) the product of the dielectric constant and the volume resistivity of the lamination adhesive is from about 0.01 to about 100 times the product of the dielectric constant and the volume resistivity of the suspending fluid;        (j) the ratio of the dielectric constant of the lamination adhesive to the dielectric constant of the suspending fluid within the temperature range of from 10 to 50° C. does not vary from this ratio at 25° C. by more than about 2 per cent;        (k) the ratio of the volume resistivity of the lamination adhesive to the volume resistivity of the suspending fluid within the temperature range of from 10 to 50° C. does not vary from this ratio at 25° C. by more than a factor of about 100;        (l) the solubility of the suspending fluid in the lamination adhesive does not exceed about 1 per cent weight/weight over the range of 10 to 50° C.; and        (m) being substantially free from mobile species.        
The aforementioned U.S. Pat. No. 6,831,769 further teaches that other problems known to occur in electro-optic displays, but which had not previously been explained, including degradation of the performance of the display with increasing temperature, even when the display is first produced, as manifested, inter alia, by a reduction in the contrast ratio of the display (the relative reflectance or optical transmission of the two extreme optical states of the display) with increasing temperature, the similar degradation of the performance of the display with increasing humidity, and the phenomenon known as “self-erasing” are all dependent upon the choice of lamination adhesive. Ota, I., et al., “Developments in Electrophoretic Displays”, Proceedings of the SID, 18, 243 (1977), describes self-erasing in an unencapsulated electrophoretic display . When the voltage applied across certain electrophoretic displays is switched off, the electrophoretic medium may reverse its optical state, and in some cases a reverse voltage, which may be larger than the operating voltage, can be observed to occur across the electrodes. It appears (although this invention is in no way limited by this belief) that the self-erasing phenomenon is due to a mismatch in electrical properties between various components of the display. Obviously, self-erasing is highly undesirable in that it reverses (or otherwise distorts, in the case of a grayscale display) the desired optical state of the display. It has been found that all of these problems may be attributable, at least in part, to changes in the electrical properties of the lamination adhesive with various environmental conditions, and that all can be reduced or eliminated by careful selection of the properties of the lamination adhesives used.
In practice, a lamination adhesive layer having a uniform thickness of 10-50 μm and a conductivity of the order of 10−10 to 10−9 S/cm has been used in electro-optic displays. This conductivity range was chosen based upon electrical models for display performance, the known conductivities of various electro-optic materials and the pixel sizes typically used in such displays. This target conductivity range actually represents a compromise. More conductive lamination adhesives are desirable because higher conductivity means the electro-optic material layer sees a larger voltage gradient. However, as the conductivity of the lamination adhesive is increased at constant pixel resolution and storage capacitance, charge supplied to a pixel is leaked to neighboring pixels; this decreases the voltage gradient across the electro-optic material and counteracts the effect of increasing the lamination adhesive conductivity. In addition, as already mentioned this lateral charge leakage may cause a change in the optical state of neighboring pixels. It is desirable that the lamination adhesive be designed to provide good lateral voltage holding capability and that the gradient in voltage across the electro-optic material layer be maximized; therefore, development performance requirements for high resolution (100 to 200 lines per inch, 4 to 8 lines per mm) active matrix displays require that isotropically conductive adhesives have conductivities in the range specified above.
Furthermore, in considering the choice of a lamination adhesive for use in an electro-optic display, attention must be paid to the process by which the display is to be assembled. Most prior art methods for final lamination of electrophoretic displays are essentially batch methods in which the electro-optic medium, the lamination adhesive and the backplane are only brought together immediately prior to final assembly, and it is desirable to provide methods better adapted for mass production. The aforementioned U.S. Pat. No. 6,982,178 describes a method of assembling a solid electro-optic display (including a particle-based electrophoretic display) which is well adapted for mass production. Essentially, this patent describes a so-called “front plane laminate” (“FPL”) which comprises, in order, a light-transmissive electrically-conductive layer; a layer of a solid electro-optic medium in electrical contact with the electrically-conductive layer; an adhesive layer; and a release sheet. Typically, the light-transmissive electrically-conductive layer will be carried on a light-transmissive substrate, which is preferably flexible, in the sense that the substrate can be manually wrapped around a drum (say) 10 inches (254 mm) in diameter without permanent deformation. The term “light-transmissive” is used in this patent and herein to mean that the layer thus designated transmits sufficient light to enable an observer, looking through that layer, to observe the change in display states of the electro-optic medium, which will be normally be viewed through the electrically-conductive layer and adjacent substrate (if present). The substrate will be typically be a polymeric film, and will normally have a thickness in the range of about 1 to about 25 mil (25 to 634 μm), preferably about 2 to about 10 mil (51 to 254 μm). The electrically-conductive layer is conveniently a thin metal oxide layer of, for example, aluminum or indium tin oxide (ITO), or may be a conductive polymer. Polyethylene terephthalate (PET) films coated with aluminum or ITO are available commercially, for example as “aluminized Mylar” (“Mylar” is a Registered Trade Mark) from E. I. du Pont de Nemours & Company, Wilmington Del., and such commercial materials may be used with good results in the front plane laminate.
Assembly of an electro-optic display using such a front plane laminate may be effected by removing the release sheet from the front plane laminate and contacting the adhesive layer with the backplane under conditions effective to cause the adhesive layer to adhere to the backplane, thereby securing the adhesive layer, layer of electro-optic medium and electrically-conductive layer to the backplane. This process is well-adapted to mass production since the front plane laminate may be mass produced, typically using roll-to-roll coating techniques, and then cut into pieces of any size needed for use with specific backplanes.
The aforementioned U.S. Pat. No. 6,982,178 also describes a method for testing the electro-optic medium in a front plane laminate prior to incorporation of the front plane laminate into a display. In this testing method, the release sheet is provided with an electrically conductive layer, and a voltage sufficient to change the optical state of the electro-optic medium is applied between this electrically conductive layer and the electrically conductive layer on the opposed side of the electro-optic medium. Observation of the electro-optic medium will then reveal any faults in the medium, thus avoiding laminating faulty electro-optic medium into a display, with the resultant cost of scrapping the entire display, not merely the faulty front plane laminate.
The aforementioned U.S. Pat. No. 6,982,178 also describes a second method for testing the electro-optic medium in a front plane laminate by placing an electrostatic charge on the release sheet, thus forming an image on the electro-optic medium. This image is then observed in the same way as before to detect any faults in the electro-optic medium.
The aforementioned 2004/0155857 describes a so-called “double release film” which is essentially a simplified version of the front plane laminate previously described. One form of the double release sheet comprises a layer of a solid electro-optic medium sandwiched between two adhesive layers, one or both of the adhesive layers being covered by a release sheet. Another form of the double release sheet comprises a layer of a solid electro-optic medium sandwiched between two release sheets. Both forms of the double release film are intended for use in a process generally similar to the process for assembling an electro-optic display from a front plane laminate already described, but involving two separate laminations; typically, in a first lamination the double release sheet is laminated to a front electrode to form a front sub-assembly, and then in a second lamination the front sub-assembly is laminated to a back-plane to form the final display.
In view of the advantages of the assembly method using a front plane laminate described in the aforementioned U.S. Pat. No. 6,982,178, it is desirable that a lamination adhesive be capable of being incorporated into such a front plane laminate. It is also desirable that a lamination adhesive be capable of being incorporated into a double release film as previously described.
This invention relates to materials and processes which are useful in the lamination and other operations in the manufacture of electro-optic displays. In particular, it has now been found that the performance of solid electro-optic displays can be improved by using a lamination adhesive having anisotropic conductivity, and in one aspect this invention relates to electro-optic displays comprising such an adhesive, to processes for forming layers of such adhesives and incorporating them into electro-optic displays, and to front plane laminates comprising such an adhesive. Since the materials and processes of the invention are especially, but not exclusively, intended for the lamination of encapsulated electrophoretic displays, they will primarily be described in connection with such displays, since any modifications and variations of the materials and processes of the present invention needed for use with other types of electro-optic displays will readily be apparent to those skilled in the manufacture of such displays.
However, the lamination adhesive is not necessarily the only polymeric component present between the electrodes in an electro-optic display. As described in, for example, the aforementioned U.S. Pat. No. 6,839,158, in an encapsulated electrophoretic display the electro-optic layer normally comprises, in addition to the electrophoretic capsules themselves, a polymeric binder which, upon drying or curing, serves to form the capsules into a mechanically coherent layer especially when, as this patent teaches is desirable, the capsules are present in the form of a monolayer of capsules. The binder is also present between the electrodes in the final display and hence also affect the electro-optic properties of the display. Indeed, to the extent that the binder lies closer to the capsules of an encapsulated electrophoretic display than does the lamination adhesive (which is typically separated from the internal phase by some thickness of binder), the binder may have a greater effect on the electro-optic properties of a display than the lamination adhesive. Similarly the continuous matrix of a rotating bichromal member medium, the continuous phase of a polymer-dispersed electrophoretic display and the wall material used in a microcell display (all of which essentially correspond to the binder of an encapsulated electrophoretic display, and all of which will hereinafter be considered binders) can have major effects on the electro-optic properties of the display. Accordingly, the present invention relates to electro-optic displays in which the properties of a binder are controlled to provide improved display performance.
The present invention thus seeks to provide an electro-optic display with a binder having optimal mechanical properties.
The present invention also seeks to provide an electro-optic display with a binder having optimal electrical properties.
In summary, the present invention seeks to provide a binder with combined manufacturing, mechanical, electrical, environmental, chemical and temporal stability properties optimally suited for use in electro-optic displays.